1. Field of the Invention
The present invention relates to electrophotographic imaging devices, and, more particularly, to a fuser assembly having a compliant end cap.
2. Description of the Related Art
An electrophotographic imaging apparatus, such as a laser printer, forms a latent image on a surface of a photoconductive material by selectively exposing an area of the surface to light. The latent electrostatic image is developed into a visible image by electrostatic toners which contain pigment components and thermoplastic components. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. A print medium (e.g., a sheet of paper) or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to a surface of the photoconductor, pulling the toner from the photoconductor onto the paper or immediate medium in the pattern of the image developed from the photoconductor. After the image is transferred to the print medium, the print medium is processed through a fuser assembly where it is heated and pressed. The fuser assembly includes a set of fuser rolls or belts, under heat, which melts and fixes the toner to the print medium surface thereby producing the printed image.
A belt fuser contains a belt whose axial location is controlled by an end cap attached to each end of a heater housing. The belt may be, for example, a polyimide tube having a Teflon® coating. The end cap has an approximate circular surface that fits inside the inside diameter of the belt to locate the belt up and down and front to back in the fuser. The approximate circular surface of the end cap is a shape to match the shape that the belt wants to take when the belt is pressed up against the heater by the back up roll. The end cap has a flange that controls the left to right axial movement of the belt. The belt is rotated by paper moving through the nip produced by the back up roller being pressed against the belt riding over a flat ceramic heater. The back up roll rotates and drives the paper. The end caps do not rotate.
There is clearance between the belt and the portion of the end cap fitting inside the belt's inner diameter so as to minimize friction between these surfaces. This clearance allows the belt axis of rotation to not be parallel to the back up roll axis of rotation. Also, the assembly of the belt and end caps may not be parallel to the back up roll axis of rotation due to manufacturing variations. Both of these effects produce a relative angle between the belt axis of rotation and the back up roll axis of rotation which causes the belt to move so that one end is pushing against the flange on the end cap. The end cap material contains glass fibers because of the load, e.g., 11 to 20 pounds, that the end cap must transmit to the back up roll to form the nip. During operation, the end of the belt wears away the plastic skin that covers these glass fibers. Once the glass fibers are exposed, the glass fibers will wear the side end(s) of the fuser belt and sometimes the side end(s) of the belt will catch on these fibers and tear. This tear causes the belt to fail and often occurs before the fuser has reached its desired life.
The relative angle between the belt axis of rotation and the back up roll axis of rotation also creates a point load. In addition to accelerated wear due to this point load, another failure mode is caused by this point load, which is a localized buckling of the fuser belt as the fuser belt contacts the end cap. This buckling usually results in the belt bending over short distances. Since it is localized the buckling fatigues the end of the belt and can put a crease in the belt. Also, in more extreme cases, due to system tolerances, the belt can have noisy dynamic buckling, which can be easily heard outside of the machine. In any case, buckling results in fatigue of the belt which results in cracks in the belt in the axial direction and circumference direction. These cracks cause failure of the belt. Also, another cause for a point load on the belt is the run out of the belt. Using coupled force transducers, a belt force oscillation on the end cap flange has been observed with the same frequency as the belt rotation.
FIG. 1 is a graph having a shaded area DF1 representing a region of no belt deformation of a prior art fuser system that does not incorporate aspects of the present invention. In FIG. 1, the X axis is the relative angle between the belt axis of rotation and the back up roll axis of rotation that is given in terms of a displacement of the AC connector end of the ceramic heater with respect to the back up roll shaft, which is called plug skew in millimeters (mm). The Y axis is the rotation of the end cap flange in degrees. As is observed from the graph of FIG. 1, the graph region below the X axis depicts a region almost completely covered with belt deformation.
What is needed in the art is a fuser assembly that reduces fuser belt deformation.